Problem: Which of these numbers is prime? ${15,\ 27,\ 54,\ 95,\ 97}$
Solution: A prime number has exactly two factors: 1 and itself. The factors of 15 are 1, 3, 5, and 15. The factors of 27 are 1, 3, 9, and 27. The factors of 54 are 1, 2, 3, 6, 9, 18, 27, and 54. The factors of 95 are 1, 5, 19, and 95. The factors of 97 are 1 and 97. Thus, 97 is a prime number.